There has been known a technique in which a mat of an insulating material is arranged between a carrier of a catalyst which is electrically energized to generate heat, and a case which contains therein the carrier of the catalyst (for example, see a first patent document). According to this mat, when the carrier of the catalyst is electrically energized, it is possible to suppress an electric current from flowing to the case.
Incidentally, moisture is contained in an exhaust gas of an engine, so water may condense in the case or the like. This water flows on an inner surface of the case, adheres to the mat, and is then absorbed into the mat. The water absorbed into the mat moves in the interior of the mat. Then, the water in the mat evaporates due to the heat of the exhaust gas or the heat of a heat generation element, so it will be removed as time passes. However, if starting and stopping of the engine are repeated in a relatively short period of time, an amount of condensed water will increase, thus making it difficult for the water in the mat to be removed. For this reason, humidity around an electrode will become high, and insulation resistance between the electrode and the case will decrease, so there is a fear that electricity may flow to the case from the electrode.